White metal crosses mark graves at the cemetery of the former Arthur G Dozier School for Boys. Photograph: /Michael Spooneybarger/Reuters

University of South Florida forensic experts announced Thursday that for the first time they have identified the remains of a boy buried at a now-closed Florida reform school where some guards were accused of brutality.

The researchers said they used DNA and other tests to identify the remains of George Owen Smith, who was 14 when he disappeared from the Arthur G Dozier School for Boys in 1940. They couldn’t say how Smith died. Official records indicated 31 burials at the school, but researchers found the remains of 55 people during a four-month excavation last year.

Researchers said Smith’s body was found in a hastily buried graved wrapped only in a burial shroud. His remains’ DNA matched a sample taken from his sister, Ovell Krell.

University officials in a press release said his mother wrote the school’s superintendent, Millard Davidson, in December 1940 asking about her son. She got a letter back saying no one knew where he was. A press conference was held Thursday to give further details.

Some former students from the 1950s and 1960s have accused employees and guards at the panhandle school of physical and sexual abuse, but the Florida Department of Law Enforcement concluded after an investigation that it couldn’t substantiate or dispute the claims. Many former Dozier inmates from that era call themselves “the White House Boys” after the white building where they say the worst abuse took place.

Researchers began last September excavating the graveyard at the school, which closed in 2011 for budgetary reasons. The dig finished in December.

The school opened in 1900 and housed over 500 boys at its peak in the 1960s, most of them for minor offenses such as truancy or running away from home.

In 1968, when corporal punishment was outlawed at state-run institutions, then-governor Claude Kirk visited and found the institution in disrepair with leaky ceilings, holes in walls, cramped sleeping quarters, no heating for the winters and buckets used as toilets.

“If one of your kids were kept in such circumstances”, he said then, “you’d be up there with rifles.”

All the bodies found were interred in coffins either made at the school or bought from manufacturers, university officials have said. Some were found under roads or overgrown trees, well away from the white, metal crosses marking the 31 officially recorded graves.

Officials have said that it’s unclear if there are other graves elsewhere on the school site. The team excavated about five acres of the property’s 1,400 acres.